Tuesday, 13 January 2015

Image of the Day: Galaxies in Collision --Preview of the Milky Way/Andromeda Destiny

The astronomers used the Atacama Large Millimeter/submillimeter Array (ALMA), and many other telescopes on the ground and in space, that gave the best preview yet of the future Milky Way/Androemda mashup: a collision that took place between two galaxies when the Universe was only half its current age. They enlisted the help of a galaxy-sized magnifying glass to reveal otherwise invisible detail. These new studies of the galaxy H-ATLAS J142935.3-002836 below have shown that this complex and distant object looks like the well-known local galaxy collision, the Antennae Galaxies.


Astronomers are now combining the power of many telescopes on Earth and in space with a vastly larger form of cosmic lens to study a case of vigorous star formation in the early Universe. Among the armada of instruments that were used to provide evidence to help unravel the mysteries of this case were ALMA, APEX, VIST, the NASA/ESA Hubble Space Telescope, the Gemini South telescope, the Keck-II telescope, the NASA Spitzer Space Telescope, the Jansky Very Large Array, CARMA, IRAM and SDSS and WISE.

The images above show the merging galaxies in the distant Universe through a gravitational lense magnifying glass. The pictures combines the views from the NASA/ESA Hubble Space Telescope and the Keck-II telescope on Hawaii (using adaptive optics). Credit: ESO/NASA/ESA/W. M. Keck Observatory.

"While astronomers are often limited by the power of their telescopes, in some cases our ability to see detail is hugely boosted by natural lenses, created by the Universe," explains lead author Hugo Messias of the Universidad de Concepción (Chile) and the Centro de Astronomía e Astrofísica da Universidade de Lisboa (Portugal). "Einstein predicted in his theory of general relativity that, given enough mass, light does not travel in a straight line but will be bent in a similar way to light refracted by a normal lens."

These cosmic lenses are created by massive structures like galaxies and galaxy clusters, which deflect the light from objects behind them due to their strong gravity - an effect, called gravitational lensing. The magnifying properties of this effect allow astronomers to study objects which would not be visible otherwise and to directly compare local galaxies with much more remote ones, seen when the Universe was significantly younger.

But for these gravitational lenses to work, the lensing galaxy, and the one far behind it, need to be very precisely aligned.

"These chance alignments are quite rare and tend to be hard to identify," adds Hugo Messias, "but, recent studies have shown that by observing at far-infrared and millimeter wavelengths we can find these cases much more efficiently."

The Hubble and Keck images revealed a detailed gravitationally-induced ring of light around the foreground galaxy. These high-resolution images also showed that the lensing galaxy is an edge-on disc galaxy - similar to our galaxy, the Milky Way - which obscures parts of the background light due to the large dust clouds it contains.

But this obscuration is not a problem for ALMA and the JVLA, since these two facilities observe the sky at longer wavelengths, which are unaffected by dust. Using the combined data the team discovered that the background system was actually an ongoing collision between two galaxies. From this point on, ALMA and the JVLA started to play a key role in further characterizing this object.

In particular, ALMA traced carbon monoxide, which allows detailed studies of star formation mechanisms in galaxies. The ALMA observations also allowed the motion of the material in the more distant object to be measured. This was essential to show that the lensed object is indeed an ongoing galactic collision forming hundreds of new stars each year, and that one of the colliding galaxies still shows signs of rotation; an indication that it was a disc galaxy just before this encounter.

The system of these two colliding galaxies resembles an object that is much closer to us: the Antennae Galaxies. This is a spectacular collision between two galaxies, which are believed to have had a disc structure in the past. While the Antennae system is forming stars at a rate of only a few tens of the mass of our Sun each year, H1429-0028 below turns more than 400 times the mass of the Sun of gas into new stars each year.

Closer to home, there is growing evidence that several million years ago the galactic center of the Milky Way was the site of another violent cosmic collision. A pair of assistant professors – Kelly Holley-Bockelmann at Vanderbilt and Tamara Bogdanović at Georgia Institute of Technology – came up with an explanation that fits these “forensic” clues, suggesting how a single event – a violent collision and merger between the galactic black hole and an intermediate-sized black hole in one of the small “satellite galaxies” that circle the Milky Way – could have produced the features that point to a more violent past for the galactic core.

The most dramatic of these extraordinary clues are the Fermi bubbles.In 2010, NASA's Fermi Gamma-ray Space Telescope unveiled a previously unseen structure centered in the Milky Way --two gamma-ray-emitting bubbles that extend 25,000 light-years north and south of the galactic center that spans 50,000 light-years and may be the remnant of an eruption from a supersized black hole at the center of our galaxy.

The structure spans more than half of the visible sky, from the constellation Virgo to the constellation Grus, and it may be millions of years old. "We don't fully understand their nature or origin," said Doug Finkbeiner, an astronomer at the Harvard-Smithsonian Center for Astrophysics, who first recognized the feature by processing publicly available data from Fermi's Large Area Telescope (LAT). The LAT is the most sensitive and highest-resolution gamma-ray detector ever launched. Gamma rays are the highest-energy form of light.

"In other galaxies, we see that starbursts can drive enormous gas outflows," said David Spergel, a scientist at Princeton University. "Whatever the energy source behind these huge bubbles may be, it is connected to many deep questions in astrophysics."

One possibility beyond the Holley-Bockelmann and Tamara Bogdanović theory includes a particle jet from the supermassive black hole at the galactic center. In many other galaxies, astronomers see fast particle jets powered by matter falling toward a central black hole. While there is no evidence the Milky Way's black hole has such a jet today, it may have in the past. The bubbles also may have formed as a result of gas outflows from a burst of star formation, perhaps the one that produced many massive star clusters in the Milky Way's center several million years ago.

The scenario began about 13 billion years ago, when the path of one of the smaller satellite galaxies orbiting the Milky Way is diverted so that it began drifting inward toward the core. According to a recent study, this may have happened dozens of times in the lifetime of the Milky Way. As the satellite galaxy – a collection of stars and gas with an intermediate-sized black hole with a mass equal to about 10,000 suns – spiraled in, most of its mass was gradually stripped away, finally leaving the black hole and a handful of gravitationally bound stars.

About 10 million years ago, the stripped down core of the satellite galaxy finally reached the galactic center. When two black holes merge, they first go through an elaborate dance. So the smaller black hole would have circled the galactic black hole for several million years before it was ultimately consumed.

As the smaller black hole circled closer and closer, it would have churned up the dust and gas in the vicinity and pushed enough material into the galactic black hole in the process to produce the Fermi bubbles.

The violent gravitational tidesproduced by the process could easily have compressed the molecular clouds in the core to the super densities required to produce the young stars that are now located on the central black hole’s doorstep.

In addition, the vigorous churning would have swept out the existing stars from the area surrounding the massive central black hole. In fact, the astronomer’s model predicts that the black holes’ merger dance should have flung a large number of the missing old stars out into the galaxy at hyper velocities, thus explaining the absence of old stars immediately around the super-massive black hole.

“The gravitational pull of the satellite galaxy’s black hole could have carved nearly 1,000 stars out of the galactic center,” said Bogdanović. “Those stars should still be racing through space, about 10,000 light years away from their original orbits.”

It should be possible to detect these stars with large surveys like the Sloan Digital Sky Survey because these stars would be traveling at much higher velocities than stars that have not undergone this type of interaction. So discovery of a large number of “high velocity stars” racing outward through the galaxy would strongly support the proposed scenario of the Milky Way and satellite galaxy merger.

The 400 by 900 light-year mosaic of several Chandra images at the top of the page shows the central region of our Milky Way galaxy, only about 25,000 light years from Earth, revealing hundreds of white dwarf stars, neutron stars, and black holes bathed in an incandescent fog of multimillion-degree gas. The supermassive black hole at the center of the Galaxy is located inside the bright white patch in the center of the image. The colors indicate X-ray energy bands - red (low), green (medium), and blue (high).

The mosaic gives a new perspective on how the turbulent Galactic Center region affects the evolution of the Galaxy as a whole. This hot gas appears to be escaping from the center into the rest of the Galaxy. The outflow of gas, chemically enriched from the frequent destruction of stars, will distribute these elements into the galactic suburbs.

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